1. Field of the Invention
This invention is generally related to amplifiers. More particularly, it is related to controlling bias current of an amplifier.
2. Related Art
With the increasing power efficiency demands from users of mobile communication devices, such as cell phones and the like, mobile communication device manufactures are continually searching for ways to improve power consumption efficiency within the mobile communication device, thereby increasing the useful operating period that a mobile communication device gets from a single charge of the power supply. During a normal voice conversation by a person using the mobile communication device, the transmitting function consumes a very large amount of available power. Therefore, transmitter energy conservation is important. One illustrative example of energy conservation in a transmitter is to configure the transmitter to transmit at a lower power level when the mobile communication device is near a base station. The transmitter is then configured to transmit at a higher power level when the mobile communication device is further away from the base station. This may be accomplished by controlling the radio frequency (RF) drive to the amplifier(s), adjusting the reference voltage and/or current applied to the amplifier bias circuit, or both. Other mobile communication devices may select the most efficient transmission frequencies under actual operating conditions. Other energy conservation techniques have focused on improving energy efficiency of the individual electrical components within the mobile communication device, such as using a high efficiency antenna system.
Conventional amplifiers employing transistors, such as bipolar transistors, heterojunction bipolar transistors or the like, draw a substantially constant collector current (IRF) through a radio frequency (RF) transistor when in operation because the reference current (IREF) and the collector voltage Vcc applied to the RF transistor are substantially constant during brief periods of operating time. However, because Vcc is typically supplied by a power supply having a limited power capacity, Vcc decreases as the power supply is discharged during device use. Ability to adjust the current IRF drawn by a reference transistor is desirable so that the quiescent current of the RF transistor (IRFO) can be varied depending upon the input applied to the amplifier. Conventional methods of adjusting IRF have employed additional circuitry and/or logic to adjust IREF (or VREF). This additional circuitry and/or logic is complex, expensive and utilizes the limited power supply. Thus, the additional circuitry and/or logic that regulates IREF (and/or VREF) increases the rate of power discharge and shortens the available useful operating life of the mobile communication device for a single power supply cycle. Furthermore, the additional circuitry is often provided on separate circuit boards or control dies, thereby requiring additional space within the device. In a mobile communication device, overall size is important as smaller devices are more desirable in the marketplace. Thus, a need exists in the industry to provide a smaller, inexpensive way of controlling the output of a power amplifier without the use of expensive additional components or sacrifices in power efficiency caused by additional control circuitry and/or logic.
Power to an amplifier may be controlled by adjusting the reference current (or reference voltage) applied to the bias circuit, that in turn sets the input terminal voltage on the input of an amplifier transistor. In a hand-held communications device, such as a cell phone or the like, the amplifier transistor is often referred to as the radio frequency (RF) transistor. Typically, power is provided by a power supply, such as, but not limited to, a battery, fuel cell and/or solar panel.
Briefly described, a current draw-away circuit employs a transistor to change the current applied to the reference transistor, that in turn sets the voltage applied to the current sinking terminal (collector) of the amplifier transistor, by diverting a portion of the reference current (or reference voltage) from the current sinking terminal of the reference transistor. Hence, the current draw-away circuit xe2x80x9cdraws awayxe2x80x9d a portion of the current sinking terminal current. Since the reference current is not directly adjusted, the need for additional complex, expensive and power consuming control circuitry is eliminated.
The current draw-away circuit is coupled to the current sinking terminal of the amplifier transistor. When in the xe2x80x9coff xe2x80x9d state, a transistor residing in the current draw-away circuit is not conductive. When in the xe2x80x9conxe2x80x9d state, the transistor residing in the current draw-away circuit is conductive and draws a draw-away current, IDRAW. The magnitude of IDRAW is determined by specifying a resistor coupled to the transistor residing in the current draw-away circuit.
In another embodiment, a device having a variable resistance is employed such that the magnitude of IDRAW can be varied in accordance with variations in resistance. In another embodiment, the magnitude of IDRAW is varied by adjusting the current applied to a control input of the transistor residing in the current draw-away circuit. The variable current is provided by a variable current source. In yet another embodiment, the magnitude of IDRAW is varied by adjusting the voltage applied to control input of the transistor residing in the current draw-away circuit. The variable voltage is provided by a variable voltage source. Another embodiment employs two or more current draw-away circuits. The draw-away circuits may be individually controlled such that one current draw-away circuit is conducting, or any number and/or any combination of current draw-away circuits are conducting.
Another embodiment employs a second transistor residing in the current draw-away circuit. When the second transistor is in the xe2x80x9coff xe2x80x9d state, the first transistor residing in the current draw-away circuit is conductive and draws a draw-away current, IDRAW When the second transistor is in the xe2x80x9conxe2x80x9d state, the first transistor residing in the current draw-away circuit is not conductive.
Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.